As is well known, many vehicles, particularly heavy trucks and trailers, utilize compressed air to operate various vehicle systems, such as braking systems, ride control systems, trailer leveling and height control systems, etc. Generally, the compressed air required for the operation of such vehicle systems is created by an air compressor carried on the vehicle. Currently, in the case of air compressors installed on North American heavy duty trucks, the inlet air for the compressor is provided by the air system from the truck engine, which inlet air typically is under turbo charged pressure. It is not uncommon for the air pressure in the intake manifolds of such trucks to be 75 p.s.i. to meet emissions requirements as established by federal regulations, and with the advent of ever higher emissions controls on diesel engines, some engine manufacturers are increasing or are expected to increase the air intake manifold pressures even higher through improvements in the turbo charger. These higher pressures have a negative impact on the performance and durability of the air compressor, which provides air to the air brake systems and other vehicle systems which utilize compressed air. A means of reducing the pressure of this inlet air is needed in order to improve compressor performance and durability.
Pressure reducing valves, which are sometimes referred to as pressure proportioning valves, are known. U.S. Pat. No. 4,438,980 to Lippiatt (“the Lippiatt patent”) discloses two designs for such valves. The first design, shown in FIG. 1 of the Lippiatt patent includes a main housing 1, having an input port 2 a pair of delivery ports 3, an exhaust port 4 and an additional vent port 5. Sealingly moveable within the housing there is provided a stepped double piston 6 having on the underside thereof an area 7 which communicates with the delivery ports 3 and which is larger than the effective area 8 on the other side of the piston which communicates with the input port 2. The two portions of the piston are provided with separate seals 9 and 10, the region between the seals 9 and 10 being vented via additional vent port 5. The piston carries a self-lapping double ended valve arrangement with respective ends 11 and 12, the end 12 being urged into engagement with a valve seat 13 by a spring 14 retained between the piston and the retaining cup 15. On downward movement of the piston the lower end of the valve member is engageable with an exhaust valve seat 16 to close off the connection between the delivery ports and the exhaust port. Further downward movement after the stabilizing effect of spring 14 is then effective to unseat the end 12 from the seat 13 thereby providing communication between the inlet port and the delivery port.
In operation of the arrangement of FIG. 1, with all ports vented, the rest position is as shown. When fluid pressure is applied to the inlet port 2, the double piston 7 moves downward to carry the double valve member toward the exhaust valve seat 16 to then close the exhaust passage and thereafter unseat valve end 12 from seat 13 to thereby provide communication from the inlet port to the delivery ports. The delivery port pressure thus increases and, upon attainment of a fluid pressure at the delivery ports which is in a reduced proportion to the inlet port pressure predetermined in accordance with the approximate ratio of the areas 8 and 7, the piston 7 returns upward to a position where the double valve laps with seats 13 and 16 closed. If the pressure at the delivery ports increases beyond the predetermined proportion sufficiently to overcome the lap stability provided mainly by spring 14 the valve end 11 unseats from the seat 16 and venting of the delivery port pressure occurs until stable equilibrium is regained. By providing the additional vent at 5, any leakage which occurs across the seal 10 or the seal 9 is vented to atmosphere.
Referring to FIG. 2, the second design again has a main housing 21, an inlet port 22, a pair of delivery ports 23 and an exhaust port 24. A piston 26 has an area 27 (generated by dimension B) subject to the pressure at the delivery ports and an annular area 28 (generated by dimension A) subject to pressure at the inlet port. The area 28 is of appreciably less area than the area 27, the ratio being such as to determine the approximate proportioning ratio of the valve. The larger diameter portion of the piston has a seal 29 and a smaller diameter portion of the piston has a seal 30 separating a region 31 which is at input port pressure from a region 32 which is always at exhaust port pressure. Carried within the piston 26 there is a sealingly slideable tubular valve member 33 with a closure surface 34 which is engageable with either or both of seat 35 formed in the piston and seat 36 formed on a tubular upstand 37 communicating with the exhaust port 24. The upstand 37 is so engageable by extending upwardly through a central aperture 38 in the piston and the valve member 33 is urged against the seat 35 by a housed spring 39. A passage in the wall of piston 26 provides a path to the valve from region 31.
In operation of the arrangement shown in FIG. 2, fluid pressure applied at the inlet port 22 is effective on the smaller area 28 of the piston 26 to tend to move the piston downward to cause the closure surface 34 to engage with seat 36 to close off the exhaust port, and further downward movement causes the member 33 to be sealingly moved in the piston to unseat the surface 34 from the seat 35 thereby providing a communication between the inlet port 22 via region 31 and the passage to delivery ports 23. Upon attainment of the predetermined proportion between the pressures at the inlet port 22 and the delivery ports 23, the valve arrangement comprising closure surface 34 and seats 36 and 35 are lapped together thereby closing off the delivery port both from the exhaust port and the inlet port. Any tendency for the delivered pressure to increase beyond the predetermined proportion of the pressure of the inlet port, to an extent to overcome the stability afforded by spring 39, causes a piston assembly to move upwards thereby briefly opening the delivery port to the exhaust port to permit partial venting to re-establish equilibrium.
While the above-described pressure reducing valves, as well as those others currently known in the art, provide the desired pressure reducing/proportioning, they sufferer from a number of disadvantages. One such disadvantage is that known valve designs are relatively complex (e.g., requiring at least two moveable parts, one or more springs, parts having complex shape, numerous seals, etc.). As such, these designs tend to be expensive to produce, complicated to assemble and prone to failure. A further disadvantage of known pressure reducing valves is that due to their complex configuration, they are not readily capable of being easily integrated into the head of the air compressor itself. Rather, they are all formed as separate units and piped in line with the air compressor, not integrally formed therewith.
What is desired, therefore, is a pressure reducing valve which reduces the air pressure of inlet air which reaches working components of a compressor, which is relatively simple in design (e.g., requiring few moveable parts, no springs, parts having simple shapes, few seals, etc.), which is inexpensive to produce, easy to assemble and not prone to failure, and which is readily capable of being easily integrated into the head of an air compressor itself.